Permanently excited synchronous motors, also referred to as PSM machines, are used on board a motor vehicle for various purposes, e.g. for a steering force assistance, a traction drive, or other drives. A PSM machine is an induction machine having a permanent magnet disposed in or on the rotor. The at least one stator comprises windings of three or more phases, and forms phases therefrom, distributed at an angle of 120°. The coils of the phases are distributed on a circumference about a rotational axis, opposite which the rotor is rotatably supported in relation to the stator.
The state variables of the PSM machine, e.g. current, voltage, flux, etc., can be plotted in the three-coordinate system (U, V, W), as is shown in FIG. 1. FIG. 1 shows an induction machine, comprising a stator (not shown) and a rotor 101, which are supported such that they can rotate in relation to one another with respect to a rotational axis 102. At least three coils are evenly distributed on the stator at 120° on a circumference surrounding the rotational axis 102. Three phases U, V, and W are provided. Each of the phases U, V, W is normally connected to the same number of coils, which are distributed on the circumference at equal spacings. The pole-pair number ZP and the number of coils connected thereto can be selected depending on the application. Preferably at least one permanent magnet 103 is disposed on the rotor 101, wherein a torque is generated when the phases U, V, W are activated with out-of-phase alternating currents, which rotates the rotor 101 about the rotational axis 102 in relation to the stator.
An out-of-phase activation of the phases U, V, W can be plotted in different coordinate systems. In stationary stator U, V, W-coordinate systems, the coordinate axes are rotated 120° in relation to one another. Because the currents of the phases U, V, W add up to zero, a current indicator, or current vector {right arrow over (I)}s can also be plotted in a two-dimensional α/β coordinate system. Furthermore, a stationary rotor d,q-coordinate system is provided in FIG. 1, the d-component of which runs such that it is rectified with the magnetic flux ΨPM of the permanent magnet 103. A q-component runs perpendicular thereto. An angle between the d axis and the α, or U axis corresponds to an electrical rotational angle θd or θel of the induction machine 100 between the rotor 101 and the stator. An electrical rotational angle Θd or Θel corresponds to the mechanical rotational angle Θmech multiplied times the pole-pair number ZP. With the transformation of the state variables into the d, q-coordinate system, the differential equations of the PSM machine are simplified, and the PSM machine can be regulated like a direct current machine. This is referred to as field oriented regulation, of FOR. With a field oriented regulation, a total target current that is to flow through the induction machine is determined in relation to a stationary rotor (flux) d,q-coordinate system, such that some control or regulating procedures can be executed more easily, and some calculations are simplified.
FIG. 2 shows the FOR of a PSM machine in an overview. A control component 205 generates a total target current Isd, Isq of the induction machine 100 d- and q-components of a voltage Usd, Usq based on the given d- and q-components. The d and q components Isd, Isq span a current vector that corresponds to the total target current. The voltage generated in the control component 205 and expressed by the d- and q-components Usd, Usq is converted from the d, q-coordinate system into a three-dimensional coordinate system, in particular the U, V, W coordinate system, by means of a converter 210. Three voltages Us1, Us2, Us3 are obtained thereby, that are converted into three corresponding pulse width modulation signals PWM1, PWM2, PWM3 by means of a vector modulator 215 on the basis of DC link voltage Udc. The DC link voltage Udc can correspond to an on-board voltage or a battery voltage when used in a motor vehicle. A pulse inverter 220 is configured to alternately connect each of the phases U, V, W to a high and a low potential of the DC link voltage Udc, such that a desired voltage is set at the phases U, V, W. The applied voltages cause actual phase currents through the phases U, V, and W. At least one actual phase current is sampled by means of a sampling device 225, which also comprises current sensors. The electrical angle Θel of the PSM is determined via a position sensor 230, based on the measured rotor position Θmech and the pole-pair number ZP, as follows:θel=Zp·θmech  (Eq. 1).
In order to compensate for the reciprocal effects of the two currents Isd and Isq, decouplings 240 can be inserted.
Aside from the induction machine 100, the pulse inverter 220, the sampling device having the current sensor(s) 225 and the position sensor 23, the depicted elements or blocks, respectively, are normally executed as method steps of a method, which runs on a processing device, which preferably comprises a programmable microcomputer. Incoming signals are normally sampled by means of analog/digital converters, and signals that are to be created are outputted either digitally, by means of a drive module, or as analog signals, by means of a digital/analog converter. Both the control device as well as the depiction of a method can be referred to in this regard.
For the execution of the FOR, in addition to the current sensors and the DC link voltage, data regarding the rotor position are also needed, in order to obtain the transformations into the coordinate system according to FIG. 1. This is important with regard to being able to determine the correct position of the permanent flux, and to obtain a precise formation of the torque with a low phase current value (and thus with high efficiency). If the position of the rotor is wrong, this can lead to the generation of a low torque, or in the worst case, to opposing torques. This is very dangerous for some applications, e.g. steering or electric mobility, etc., because in these cases, instead of a machine being accelerated in one direction, it could be stopped, and moved in the other direction, which could lead to accidents. For this reason, a position sensor (or rotational rate sensor) for measuring the position of the rotor is very important, and indispensable.
The functionality of the position sensor can, however, be compromised by various effects. With a total malfunction of the sensor, or with a partial malfunction, e.g. failure of a channel of the sensor, the position of the sensor cannot be determined or correctly determined. Furthermore, a displacement of the sensor on the shaft, a common error, can lead to an offsetting of the mechanical angle of the rotor, which would result in an erroneous coordinate system, and thus to an uncontrolled state.
A redundant sensor can be installed for monitoring the functionality of the position sensor, such that the two sensors monitor each other. This however increases the costs for the overall system. Alternatively, computer models can be used, for cost reduction purposes as well, that determine the position, which is then compared with the measured sensor position. There are numerous published (e.g. Prof. Dr.-Ing. Dierk Schröder: “Elektrische Antriebe Regelung von Antriebssystemen” [“Electrical Drive Regulation of Drive Systems”], 3rd ed. Springer Publishing, TU Munich, 2001) methods for determining the position (Matsui, Wallmarkt, Leonbergerbeobachter, Kalmannfilter, . . . ). These methods are based on EMF (Electro-Magnetic Force), i.e. they require the voltage of the machine induced by the permanent flux and the rotational rate for the calculation thereof. With low rotational rates, and at a standstill, the EMF is very low, and provides no significant results. For this reason, this method is first used above a certain minimum rotational rate. This minimum rotational rate depends on the drive and the sensor system. It normally lies in a range of 10% to 15% of the maximum rotational rate. For lower rotational rates than this minimum rotational rate, the determination of the rotor position with this method may be erroneous. In accordance with the prior art, injection methods, which implant signals with a higher frequency into the machine, can determine the position within this low rotational rate range. The acoustic problems, which cause noises in the current through high frequencies, are disadvantageous.